A “coherent” optical transmission system is characterized by its capability to perform coherent detection. Two optical signals are coherent when the phase difference between their waves is constant, they are non-coherent if there is a random or changing phase relationship rapidly. One type of coherent signals are in-phase/quadrature signals. The “quadrature” signal differs in phase by 90 degrees from the “in-phase” or reference signal. The in-phase signal is referred to as “I,” and the quadrature signal that is shifted by 90 degrees is called “Q.” Coherent detection means that an optical receiver can track the phase of an optical transmitter (and hence “phase coherence”) so as to extract any phase and frequency information carried by a transmitted signal.